1. Field of the Invention
This invention relates to a fixture for fixing a semiconductor device such as a semiconductor package, which contains a semiconductor element, for example, to a radiator such as a heat sink, and an assembly of the semiconductor device.
2. Description of the Related Art
In general, a semiconductor device of this type is configured so that a frame member is mounted on a surface of a baseplate and a semiconductor element is contained within the frame member. Connection terminals for external connection protrude from peripheral walls of the frame member, and a lid is put on that opening of the frame member which is distant from the baseplate, whereby the opening is closed.
The semiconductor device constructed in this manner is used with its connection terminals electrically connected to a circuit of a printed wiring board. During this use, the semiconductor element contained in the semiconductor device is heated, so that the temperature of the device increases. Since the semiconductor device is degraded in performance by the temperature increase, produced heat should be discharged to the outside to keep the temperature at a tolerance level or below.
Accordingly, the semiconductor device is fixed to a radiator by screws through through-holes at four corners of the baseplate. Thus, the heat from the semiconductor device can be transferred to the radiator through the reverse surface of the baseplate, whereupon the semiconductor device is cooled.
If this mounting structure is used, however, it is difficult to bring the entire reverse surface of the baseplate uniformly into contact with the radiator. If the baseplate is warped, for example, the thermal contact resistance increases in a region other than its four corners, near the center in which the frame member is mounted. This thermal contact resistance can be made uniform by enhancing the manufacturing accuracy of the baseplate including the amount of warp. If this is done, however, machining the baseplate is very troublesome and entails high costs.
On the other hand, Jpn. UM Appln. KOKAI Publication No. 3-59688 (Patent Document 1) discloses a technique such that a wall surface of a semiconductor element is brought into pressure contact with a radiator plate by means of, for example, an L-shaped metal fitting, whereby the thermal contact resistance between the wall surface and plate is reduced. The L-shaped metal fitting of Patent Document 1 has its one end passed through a hole in a backing and the other end in contact with the wall surface of the semiconductor element. The metal fitting is fastened to the base member by screws that are passed through threaded holes between a bent portion and the other end.
Also disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2004-288949 (Patent Document 2) is a structure in which a baseplate of a semiconductor device is fixed to a radiator under pressure with a graphite sheet with high thermal conductivity between them. By using this mounting structure, the manufacturing accuracy of the baseplate and the thermal contact resistance can be reduced without enhancing the flatness of the baseplate surface.
Alternatively, a structure may be provided such that grease for heat radiation is applied in place of the graphite sheet between the baseplate and radiator.
In the mounting structure disclosed in Patent Document 1, however, the semiconductor device is pressed against the radiator by means of the L-shaped cantilevered metal fitting. Therefore, it is difficult to uniformly press the wall surface of the semiconductor element against the radiator plate, so that the thermal contact resistance cannot be sufficiently reduced. If this mounting structure is used, moreover, the semiconductor element and L-shaped metal fitting must be held down by hand as they are fastened, so that the mounting operation is laborious. If this structure is used, furthermore, the installation area of the semiconductor element including the L-shaped metal fitting becomes greater than a thermal contact area between the semiconductor element and radiator plate, so that the mounting layout is restricted to that extent, and the device configuration is inevitably enlarged.
In the mounting structure disclosed in Patent Document 2, on the other hand, the thermal contact resistance between the semiconductor device and radiator can be made lower than in the case of Patent Document 1. Since the graphite sheet or grease for heat radiation is interposed between the device and radiator, however, the electrical resistance is higher to that extent. Thus, it is hard to use the package itself of the semiconductor device as an electrode.
If the mounting structure of Patent Document 2 is used, moreover, so many components are required that the assembly and manufacture are troublesome and the product costs are high.